Tightening the nitrogen loop in the Mackenzie Basin
A new catch crop project in the Ahuriri catchment is bringing farmers, scientists and us together to explore smarter nitrogen (N) management in winter forage systems.
Smarter nitrogen use
Led by the Bioeconomy Science Institute (Plant and Food Research Group) and supported by us, the trial sites are situated on three farms within a 25-minute radius of Omarama.
The goal is understanding how nitrogen moves through winter grazing systems and how catch crops and improvements in fertiliser management might help reduce the risk of nitrogen loss to the environment.
We have begun a focused programme of work in the Ahuriri catchment, to protect the ongoing health of Lake Benmore as well as working with landowners to protect wetlands, enhance biodiversity and identify critical habitats in the area.
Reducing and mitigating nitrogen lost to groundwater is a key aspect of the work, as nitrogen is one of the nutrients that has had an adverse effect on the health of Lake Benmore’s Ahuriri Arm and its tributaries.
Catch crops in action
Dr Brendon Malcolm, team leader soil function and health at Bioeconomy Science Institute, said working closely with farmers is at the heart of the project.
“We want to understand local production systems and fertiliser practices, share what we know and see if catch crops can offer practical value – not just environmentally, but economically too.”
Catch crops – like oats or short-rotation ryegrass – are fast-growing species that mop up nitrogen left behind in the soil following winter grazing of forage crops. This reduces the risk of it leaching into groundwater and losses of nutrients and sediment to overland flow.
Protecting Lake Benmore
The Mackenzie Basin trials are an exciting opportunity to trial catch crops at high altitude under colder winter/spring conditions.
Crops like oats have a low base temperature for emergence (about 2°C) and leaf appearance (about 3°C). As long as daily average soil temperatures are consistently higher than 3°C, ideally 4°C, these crops can establish and take up nitrogen.
“While the crops can be slow to get going, they take up nitrogen over and above their requirements for growth (known as luxury nitrogen uptake) in early growth stages and most of the crop biomass is put on in late spring," said Brendon.
“For example, when only about 10–20 per cent of the biomass has accumulated approximately 50 per cent of the total nitrogen uptake has been captured by the plants.
Shane Maley (lead field researcher, Bioeconomy Science Institute) installing a CropX sensor
“They are also quick at putting root systems down – in early growth stages, roots will be developing three times faster than the above-ground growth.”
The Mackenzie Basin trials will run across two seasons, with soil and crop testing before grazing, after grazing and at key growth stages during the catch crop phase. These simple proof-of-concept trials will help show what’s possible in the Basin’s cold, high-country conditions.
“We are excited to keep pushing the boundaries of where we can take catch crops,” said Dr Brendon Malcolm.
Shane Gilmer, principal implementation advisor water and land, said the council is excited by the collaboration and what it could mean for the future.
“This is about putting good science into farmers’ hands, so they can make decisions that benefit both their business and the environment. It’s the kind of partnership approach we want to see more of across the region,” Gilmer said.
You can follow the Bioeconomy Science Institute team’s catch crop journey at Catch Crops for Cleaner Water on Facebook
Find out more about the work we are undertaking alongside landowners in the Ahuriri Catchment.